1. Field of the Invention
The present invention relates to a matrix type display device, an electronic system including the same and a method of driving such a display device. Particularly, the present invention concerns a matrix type display device which can display characters and icons.
2. Description of the Related Art
As for a simple matrix type display device, a built-in RAM is known. In such a display device, the display is carried out by transferring display data for one image from the CPU or the like to the built-in RAM, and sequentially reading display data for one scan line from the built-in RAM. If the image on display remains the same as in a still image, the display data transfer from the CPU is unnecessary and the display operation can be carried out only by the display data from the built-in RAM, thus reducing the power consumption.
Another matrix type display device with a character pattern generator is known. In addition to the built-in RAM in such a matrix type display device, the display is performed by transferring character codes for one image from the CPU or the like to the built-in RAM, and converting the character codes into dot image display data through the character pattern generator. This matrix type display device can operate with a reduced power consumption and be controlled easily. Therefore, such a matrix type display device has been broadly used in many portable electronic devices such as portable telephones, electronic pocketbooks and others.
Such a matrix type display device with a character pattern generator is required to provide a function of displaying icons such as indicators, symbols and others in addition to the characters. If such icons can be displayed, for example, a portable telephone can display an indicator showing the residue of batteries, the radio field intensity, a symbol representing a telephone or the like.
FIG. 13 shows a matrix type display device with a character pattern generator which is constructed in accordance with the prior art and which can display icons. The matrix type display device comprises a matrix panel 116, a scan electrode driving circuit 110, a signal electrode driving circuit 111, a display signal transferring circuit 126 and a display signal generating circuit 140. The matrix panel 116 includes display pixels arranged in a matrix, and a plurality of signal electrodes crossing a scan electrode, the number of dots being n.times.(m+1). Over the matrix panel 116, a character display area 142 (which includes crossing areas between signal electrodes S1-Sn and scan electrodes C1-Cm) and an icon display area 144 (which includes crossing areas between the signal electrodes S1-Sn and a scan electrode CS) are provided. For example, if one character is to De displayed by 5.times.7 dots, it is possible to display characters equal to (n/5).times.(m/7) as well as icons equal to n. The display signal generating circuit 140 includes a display code memory 114 for storing display codes for one image(display image), a character pattern generating circuit 113 for generating a character pattern of dot image for the display codes, an icon display memory 115 for storing an icon pattern of dot image and a multiplexer 112 for multiplexing the output of the character pattern generating circuit 113 and the icon display memory 115 to form an output display signal 123. The display signal transferring circuit 126 transfers the display signal 123 to the signal electrode driving circuit 111. The signal electrode driving circuit 111 and the scan electrode driving circuit 110 form signals for driving the signal electrodes and the scan electrodes, respectively.
The operation of the prior art is explained hereafter. The characters are displayed through the following operation. First, character codes of a character to be displayed are written in the display code memory 114 at a desired address layout. The display code memory 114 is not rewritten unless any changes are made to the displayed image, and thus reducing the power consumption. Then a character code signal 120 is read out from the display code memory 114 responding to a read-out signal 118, and is transferred to the character pattern generating circuit 113. The character pattern generating circuit 113 responds to the character code signal 120 to generate a character pattern display signal 121 which is transferred to the signal electrode driving circuit 111 through the multiplexer 112 and display signal transferring circuit 126.
On the other hand, icons such as indicators, symbols and others are displayed by the following manner. First, a pattern of dot image for an icon to be displayed is written in the icon display memory 115 through the CPU or the like. The icon display memory 115 then responds to a read-out signal 118 for an icon pattern display signal 122 to be read out therefrom. The icon pattern display signal 122 is then transferred to the signal electrode driving circuit 111 through the multiplexer 112 and the display signal transferring circuit 126.
The multiplexer 112 responds to a select signal 119 to select either the character pattern signal 121 or the icon pattern display signal 122. The selected signal, signals 121 or 122, is multiplexed to be a display signal 123. In other words, the signals 121, 122 are transferred as the display signal 123 to the display signal transferring circuit 126 through the multiplexer 112 within one horizontal period in the time division manner. The display signal transferring circuit 126 accumulates the display signal 123 as data for each pixel line and transfer the data to the signal electrode driving circuit 111. Such a transfer is carried out for every horizontal period. The signal electrode driving circuit 111 outputs liquid-crystal drive voltages corresponding to the transferred display signal to the signal electrodes S1-Sn. On the other hand, the scan electrode driving circuit 110 responds to a scan control signal 117 to scan the scan electrodes C1-Cm and CS sequentially for each horizontal period. In other words, the display signal transferring circuit 126 receives and stores the display signal 123 for the scan electrode C1 before the scan electrode C1 is scanned. As the scan electrode C1 is scanned, the circuit 126 outputs liquid-crystal drive voltages corresponding to the stored display signal 123 to the signal electrodes S1-Sn to display one pixel line. During this horizontal period, the circuit 126 receives another display signal 123 for the next scan electrode C2. Since the scan electrode CS is only used to display icons, the display signal 123 for the scan electrode CS includes only the icon pattern display signal 122, but not the character pattern display signal 121. In such a manner, the sequentially scanned scan electrodes and the signal electrodes to which the drive voltages corresponding to the display signal are applied to perform together the display operation in the matrix type display device.
However, the aforementioned matrix type display device of the prior art has many disadvantages.
First, the prior art has a problem in that the processing in the display signal generating circuit 140 becomes complicated with an increased processing time. More particularly, in the prior art, the external CPU or the like must write character codes into the display code memory 114 and also dot images into the icon display memory 115. Therefore, the CPU is required to handle a combined data of character codes and dot images. This makes the process complicated and increases the burden on the CPU. It is further assumed that the data bus from the CPU is eight-bit data bus. In such a case, since one character can be specified by an eight-bit character code, only one data transfer from the CPU is necessary to display one character (e.g., 5.times.7 dots). On the contrary, the data of an icon must be transferred as a dot image. For example, if an icon of 5.times.7 dots is to be displayed, four or five data transfers are required, increasing the processing time.
Second, the prior art has a problem in that the icon display area is not freely arranged on the matrix panel 116 under the limitations of arrangement with respect to the scan electrodes and signal electrodes. For example, it an icon display area is to be provided on the right side of the character display area 142, it is necessary to extend the icon scan electrode CS, and display the icon at the areas where the extended icon scan electrode CS and the signal electrode Sn cross(see FIG. 12). Since the signal electrodes and scan electrodes are formed on the same substrate, they cannot cross one another. Therefore, a pattern of extending the scan electrode CS becomes complicated in addition to increase in the length of the extended scan electrode CS. The complicated electrode extending pattern makes the design of the matrix panel 116 difficult while the further extended scan electrode increases the parasitic resistance, and thus degrading the quality of display. Furthermore, the prior art cannot substantially display the icon in the character display area 142.
To overcome such a problem, it is possible to provide another icon display signal electrode SS on the matrix panel 116. According to such a measure, an icon can be displayed at crossing areas between the signal electrode SS and the scan electrodes C1-Cm and CS. However, providing such a signal electrode SS raises another problem in that the data transfer from the multiplexer 112 to the display signal transferring circuit 126 becomes complicated. More particularly, the character pattern generating circuit 113 usually outputs display signal (five-bit) for one character at the same time. Nevertheless, it the icon display signal electrode SS is newly provided and when display signal is to be transferred to dots on the scan electrodes C1-Cm, the multiplexer 112 must repeatedly output character display five-bit signal (n/5) times within one horizontal period before an icon display one-bit signal is output. On the other hand, when display signal is to be transferred to dots on the scan electrode CS, the multiplexer 112 must output icon display one-bit signal (n+1) times. Thus, if the icon display signal electrode SS is newly provided in the prior art, the multiplexer 112 must perform a different multiplexing operation for each scan line and handle a very complicated process.
Since the character and icon display areas are fixed in the prior art, those users who design portable telephones or the like using the matrix type display device can not rearrange freely the character and icon display areas.
Third, the prior art raises a still another problem in that it may degrade the quality of display depending on the rate of lighting-on in the character patterns. Namely, the prior art may create a shadow phenomenon (cross-talk). Such a problem is raised not only in the aforementioned matrix type display device of the prior art, but also generally in all the matrix type display devices with character pattern generators.